snownskate69
Posts: 362
Joined: 7/17/2002
From: Auburn,
AL, USA
Status: offline
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AMT pretty much has it. Basically, the engine drives a vacuum pump which, as it's name implies, sucks air through tubing set up in the aircraft. The gyros have blades that catch the air and force it to turn. It's kinda like a paddle boat, the ones you pedal. If you get going real fast and then take your feet off of the pedals, they will still turn because of the water moving over the paddle wheel type drive they use. It's the same idea, only with air. As far as how it works goes:
If you've ever spun up a toy gyro or used a top, the property that they work on is evident. A top, even though spun slowly, will stay nearly vertical. If you spin it real fast it will hardly have any wobble at all. Spin it even faster in an aircraft ( the aircraft gyros can run up to 18,000 rpm), and it will stay perfectly vertical, at 90 degrees to gravity, which is down. If you ran a toy top on some cardboard, picked up the cardboard and tilted it slightly, it would still point straight down. If you tilted it at 45 degrees the top would still point straight down, but it wouldn't ever stay on the cardboard, so that demo won't really work. Knowing that a gyro will stay fixed in position allows you to do many things. In the aircraft world, if you fix an object to it ( like a compass card or a artificial airplane) and have it up against a scale, the gyro will remain relatively fixed in it's natural position, while the scale and the actual airplane move around it. This is why if you pitch up, the little artificial airplane will stay in one place(its attached to the airplane) while the scale (which in this case is a line that represents the horizon, as well as degree marks for bank and pitch) will move around it, because the airplane is no longer 90 degrees to the surface of the earth(gravity). When a force(including friction) acts on a gyro, the effect is felt 90 degrees from that force. It is that property and rigidity in space, which is what I just tried describing, that gyros operate on. The instruments in an aircraft which run on gyros are:
Artificial horizon
directional gyro (heading indicator)
Turn coordinator (usually electric and not vacuum, for redundancy)
If you don't know what they look like, just type those in a google image search and you will come up with them.
Precession causes very few errors in the artificial horizon, however because of the axis that the heading indicator works on, it does cause a slight drift in the heading. As amt said, the rule of thumb is to reset the heading indicator every 15 minutes.
Another limitation is tumbling. Tumbling is when the pitch and bank angles of the instrument are exceeded. It is usually 60 degrees of pitch and 100 degrees of bank. When this happens, the gyros will tumble to re align themselves with the axis they are supposed to be running on, which it is now easier to get to by flipping over. This causes extreme errors in the instruments before they spin up and realign themselves, however significant adjustment to the heading indicator may still be needed. Aerobatic planes have gyros which can be "caged", limiting thier travel and avoiding the issue. Tumbling gyros also tends to decrease thier lifespan.
Hope this helps
Alex
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Aircraft Gyroscopes - 
